A conventional decanter centrifuge for the above defined purpose usually comprises a rotatable drum and a sludge conveyor arranged therein. The sludge conveyor is rotatable at a speed differing from that of the drum and is adapted to convey the formed sludge axially towards a number of outlet openings in the drum. The sludge conveyor may have one or more conveying members extending helically and continuously several turns around the rotational axis of the drum.
It has proved difficult by means of a conventional decanter centrifuge of this kind to compress the sludge formed at the surrounding wall of the outer drum as effectively as would be desired and, simultaneously, to accomplish an axial transportation thereof in the drum. Thus, owing to the axial transportation of the sludge, an agitation thereof occurs, so that it is remixed with the liquid continuously flowing through the drum in a direct contact with and radially inside of the sludge. Hereby, the desired departure of liquid from the sludge is not obtained and the sludge, therefore, when leaving the decanter centrifuge has a too low dry substance content. The above discussed problem can be resolved to some extent by reduction of the rotational speed difference between the drum and the sludge conveyor and by reduction of the amount of suspension, e.g. sewage water, that is supplied to the decanter centrifuge per unit of time. However, this means that the decanter centrifuge gets a strongly reduced capacity. Furthermore, it is required in connection with increased dryness of the sludge an increased force for the axial transportation of the sludge within the drum, i.e. the momentum to which the sludge conveyor has to be subjected and which it has to transfer will be larger. These circumstances put limits to how this method of resolving the problem can be used in practice. In DE-33 01 099 C2 it has been proposed that the sludge conveyor in addition to its sludge conveying member should be provided with plates arranged to accomplish cutting surfaces in the separated and compressed sludge for facilitating that the sludge is freed from liquid. As far as known this proposal has not been used in practice to any substantial degree, if any. Besides, it is not believed that this would constitute any solution to the problems discussed above.
In U.S. Pat. No. 2,862,658 it has been proposed that separated sludge should be constantly subjected to stirring or agitation during its axial transportation in a decanter centrifuge to facilitate for a certain relatively light liquid component to leave the sludge. Such a constant agitation of the separated sludge would directly counteract, however, the above said desideratum of accomplishing a sludge as dry as possible, e.g. in connection with cleaning of sewage water.
In a presentation "Improved Sludge Dewatering with a new Decanter Centrfiuge" made by H. Reinach & W. Stahl at a "Filter Conference" in Karlsruhe, Germany, in the year 1989, it has been suggested that separated sludge would be effectively dewatered while conveyed in the conical part of a decanter centrifuge, i.e. radially inside the level of the separated liquid in the decanter centrifuge, if the sludge conveyor were provided with axial rows of inclined separate plates or blades instead of a helically extending conveying member. It is thus suggested that the blades should give the sludge the form of a ridge, which gradually and intermittently is displaced axially from one of its sides by the separate blades, across the longitudinal direction of the ridge, and that liquid would be given a possibility to drain off from the sludge not only through the front side slope of the ridge but also through the rear side slope of it during the periods of time when successive portions of the last mentioned slope is free from contact with the blades. Such a technique could probably not be used, giving the desired result, in the part of the decanter centrifuge wherein the sludge is situated in liquid, i.e. radially outside the cylindrical liquid surface formed in the decanter centrifuge.